The crystal structures and molecular stereochemistries of biologically significant iron porphyrins are to be determined by applying the techniques of X-ray diffraction analysis to the three- dimensional data afforded by single crystals. Precisely measured intensity data of the widest practicable scope from the best preparable of specimen crystals are to be employed for structure analyses. Of primary interest for these analyses are iron porphyrins that have been modified (by Dr. James P. Collman's research group) to serve as model systems for the active sites in the oxygen-carrying hemoproteins and in the cytochrome P450 based mixed-function oxygenases. Other metalloporphyrins may also be subjected to structure analysis.